In drivetrain systems where the coupling between the engine and the torque converter is divided into two inertia masses, one connected with the engine and impeller and the other connected to the input shell or cover of a torque converter, an input isolator assembly is generally employed to transmit power between the inertia masses while damping torsional vibrations. The input isolators typically include a spring means.
Current automatic transmissions for vehicles typically employ not only a selectively engageable friction clutch but also an isolator assembly to connect the torque converter impeller and the torque converter turbine, thereby improving the efficiency of the drivetrain. One difficulty with such systems is that engaging the clutch at low engine speeds sometimes results in vibrations in the drivetrain which are perceptible by the driver. The engagement speed range for such arrangements is, therefore, limited.
Drivetrain designers have attempted to lower the engine speed at which the clutch can be engaged without generating the undesirable vibrational disturbances. Some designs have incorporated a viscous slipping clutch which is disposed in series with a friction clutch. This arrangement has the benefit of permitting lower engagement speeds, but it does increase the cost of the system.
Other designers have suggested dual mass flywheel systems to reduce drivetrain disturbances in countershaft type transmissions with synchronizer shifting controls. In such designs, the engine and transmission are completely disconnected during the ratio interchanges by a selectively engageable clutch member. These systems provide some improved drivetrain damping, but they increase the number of spring systems or friction interface connections, thus becoming relatively complex and concomitantly more costly.
One pertinent arrangement is disclosed in U.S. Pat. No. 5,121,821 to Poorman et al., which patent is owned by the assignee of the present invention. The Poorman disclosure is directed to a drivetrain coupling between the engine and the torque converter that permits the friction clutch to be fully engaged at lower engine speeds, while avoiding unpleasant disturbances in the drivetrain. To accomplish this goal, the inertia mass of the torque convertor, torque convertor cover and clutch is separated from the engine inertia mass and, along with a portion of the flywheel inertia, is added to the transmission inertia mass. By separating these inertias, the natural frequency of the system can be designed to a lower value, thereby permitting engagement of the clutch at lower engine speeds. Moreover, the isolator can be removed from the clutch for placement between the inertia masses. This design has been effective and generally accomplishes its intended goal.
The present invention contemplates a further improvement over the device as described in the aforesaid U.S. Pat. No. 5,121,821.